I just finished the story myself. Good emotional ending. And I loved the kinds of puzzles that the game throws at you towards the end. They're all on the mild side and all involve touch-screen interaction. Basically, my favorite kinds of Layton puzzles.
Better than Diabolical Box? Maybe. I'll save that judgment for when I complete most of the bonus content. Though, in terms of where I am after completing the story, I can say that both games are at least even with each other. Layton 3's story and puzzles click a bit more with me, but the feeling of the series losing its freshness is definitely there.
Honestly, even though Layton 1 has been outdone in some ways with these successive titles, I'd still say that it was the only one of the three that I'd place in the Handheld GOTY category. It really smacked me in the face with its goodness and I loved the presentation. Since then I've been desensitized to the style of the games, so they don't hit me as hard. But Level 5 hasn't kept all three localized Layton titles from being compelling, and that's why I keep coming back. The puzzles are fun, the presentation and art direction are top notch, interesting, and unlike anything else I've played, and I just love Professor Layton and Co. as characters.
Working my way towards 100% (which I never did achieve with Diabolical Box, though I got close) will likely give me my fill of Layton for a while, but I'm still looking forward to the prequel trilogy and beyond. This is a series that I'm sticking to.